In recent years, transmitting not only speech data but also large volume data such as still image data and moving image data has become common along with the increasing adoption of multimedia-enabled information in cellular mobile communication systems. Meanwhile, studies have been actively carried out to achieve high transmission-rate communication using a wide radio band, multiple-input multiple-output (MIMO) transmission technology, and interference control technique in long term evolution advanced (LTE-Advanced).
In addition, studies have been carried out on achieving a high transmission rate at hotspots through deployment of small cells, each being a radio communication base station apparatus (hereinafter, abbreviated as “base station”) using low transmission power in cellular mobile communication systems. Allocating a frequency different from that for macro cells as a carrier frequency for operating small cells has been also under study. A high frequency such as 3.5 GHz has become a candidate. When small cells and macro cells are operated using different frequencies, transmission signals from macro cells do not interfere with communication performed by small cells. Accordingly, the deployment of small cells can achieve high transmission-rate communication.
In LTE-Advanced, a cell-specific reference signal (CRS) or a demodulation reference signal (DMRS) is used as a reference signal (RS) used for demodulating a physical downlink shared channel (PDSCH), which corresponds to a data signal.
CRSs are also used for channel quality measurement in addition to demodulation of data signals while the number of antenna ports and the resource positions for CRSs are determined on a per cell basis. For this reason, it is difficult to change the amount of resources for the CRSs for each radio communication terminal apparatus (hereinafter, abbreviated as “terminal,” which may be also called a user equipment (UE)).
Meanwhile, the number of antenna ports and the resource positions for DMRSs are determined on a per user basis, and DMRSs are mainly used for demodulating data signals. In addition, DMRSs mapped in resource block (RB) pairs for a different terminal (to be described, hereinafter) have no effect on signal assignment. For this reason, it is easier to optimize the amount of resources for DMRSs for each terminal.
Small cells provide coverage for low-mobility terminals and indoor terminals with a small delay spread, supposedly. The channel quality of these terminals is expected to be good. In this respect, studies have been carried out on further increasing the transmission rate through DMRS reduction for terminals having good channel quality and use of the resources that have become available as a result of reducing DMRSs, as a data region (see, Non-Patent Literatures (hereinafter, referred to as “NPL”) 1 and 2).
(Explanation of Resources)
In LTE and LTE-Advanced, one RB consists of 12 subcarriers in the frequency-domain and 0.5 msec in the time-domain. A resource unit formed by combining two RBs in the time-domain is called an RB pair. Accordingly, an RB pair consists of 12 subcarriers and 1 mesc. An RB pair may be simply called an RB when the term is used for representing a group of 12 subcarriers in the frequency domain. In addition, an RB pair is called a physical RB (PRB) in the physical layer. Moreover, the first-half RB (0.5 msec) of a PRB pair is called a first slot, and the second-half RB (0.5 msec) of the PRB pair is called a second slot.
In addition, a unit consisting of one subcarrier and one OFDM symbol is called a resource element (RE). The number of OFDM symbols per RB pair varies depending on the CP length of OFDM symbols. In the case of normal CP, each RB pair includes 14 OFDM symbols. In the case of extended CP, each RB pair includes 12 OFDM symbols.
FIG. 1 illustrates a DMRS mapping pattern in the case of normal CP. When only antenna ports #7 and #8 are used, only 12 REs are allocated to DMRSs. When antenna port #9 is used at least, 24 REs are allocated to DMRSs. When antenna ports #7, #8, #9 and #10 are used, antenna ports #7 and #8 are CDMA multiplexed by means of orthogonal cover codes (OCCs) on the adjacent OFDM symbols of the same subcarrier and antenna ports #9 and #10 are CDMA multiplexed by means of OCCs on the adjacent OFDM symbols of the same subcarrier. Moreover, when antenna ports #11, #12, #13, and #14 are used, antenna ports #7, #8, #11, and #13 are CDMA multiplexed by means of orthogonal cover codes using four REs of the same subcarrier and antenna ports #9, #10, #12, and #14 are CDMA multiplexed by means of OCCs using four REs of the same subcarriers.
Multiple antenna ports are used in single user MIMO (SU-MIMO) and multi user MIMO (MU-MIMO). In SU-MIMO, antenna ports #7 to #14 can be used for a single terminal. However, only antenna ports #7 and #8 can be each used as a single antenna port, and for the number of antenna ports X (>1), antenna ports #7, #8, . . . #(X+6) are used. For example, when the number of antenna ports is 6, antenna ports #7, #8, #9, #10, #11, and #12 are used. MU-MIMO based on orthogonal multiplexing is achieved by multiplexing antenna ports #7 and #8 by means of OCCs. However, each terminal is only aware of allocation for the terminal and thus cannot know whether or not MU-MIMO is actually performed.
(Reduction in Frequency-Domain Direction)
FIGS. 2A and 2B illustrate an example of a DMRS mapping pattern in which DMRSs are reduced in the frequency-domain direction. Assigning this mapping pattern to a terminal in a reception environment where the change in channel quality in the frequency-domain is moderate, such as an environment where a terminal is located indoors and with a small delay spread can minimize the degradation of reception quality due to the reduction of DMRSs. For reducing DMRSs in the frequency-domain direction, multiplexing is performed using four REs of the same subcarriers. Thus, CDMA multiplexing of antenna ports #7, #8, #11, and #13 and CDMA multiplexing of antenna ports #9, #10, #12, and #14 can be supported.
(Reduction in Time-Domain Direction)
FIG. 3 illustrates an example of a DMRS mapping pattern in which DMRSs are reduced in the time-domain direction. Assigning this mapping pattern to a low-mobility terminal in a reception environment where the change in channel quality in the time-domain is moderate can minimize the degradation of reception quality due to the reduction of DMRSs. However, when antenna ports #7 to #14 are used, antenna ports #7, #8, #11, and #13 are to be CDMA multiplexed using four REs of the same subcarrier, and antenna ports #9, #10, #12, and #14 are to be CDMA multiplexed using four REs of the same subcarrier. Accordingly, antenna ports #11 to #14 cannot be supported by the current design.